1. Field of the Invention
This invention relates to photosensitive materials for electrophotography which make use of organic photoconductive compounds or substances dispersed in a specific type of binder polymer and which are particularly suitable for use in electrophotographic systems using a charging/exposing/developing process.
2. Description of the Prior Art
As is known in the art, photosensitive compounds or substances which have been used in the field of electrophotography are broadly classified into inorganic photoconductors and organic photoconductors. Typical examples of the inorganic photoconductor include selenium, zinc oxide, titanium oxide, cadmium sulfide and the like. Typical examples of the organic photoconductors include phthalocyanine pigments, diazo pigments and the like.
The photosensitive material using the inorganic photoconductor is not always satisfactory with respect to the thermal stability and the durability. With inorganic photoconductors which are, in most case, toxic, there are problems on their preparation and handling.
On the other hand, the photosensitive material using the organic photoconductor is free of any toxic problem with excellent productivity and good economy. In addition, organic photoconductors can be arbitrarily controlled in photosensitive characteristic by modification or proper designing of the molecule. Accordingly, developments of organic photoconductors have been extensively made and a number of organic photoconductors have been in use. In fact, organic photoconductors are now being predominantly employed in the field of electrophotography.
Known organic photosensitive materials have usually a double-layer structure which includes a carrier generation layer capable of absorbing light to generate carriers and a carrier transport layer wherein the generated carriers are transported. Many attempts have been made for higher sensitivity. For attaining high sensitivity in the double-layer structure, the usual practice is that the carrier generation layer is formed in a thickness of several micrometers and the carrier transport layer is formed in a thickness of several tens micrometers. In order to impart high strength and good printing resistance, it is general that the carrier generation layer is formed directly on a substrate and the carrier transport layer is formed as a surface layer. The carrier generating agent used is one which works through movement of positive holes and the resulting photosensitive material is eventually of the negative charge type.
The negative charge systems, however, have several drawbacks: (1) negative charges attack oxygen in air to produce ozone which is not only harmful to human body, but also shortens the life of the photosensitive material by reaction therewith; (2) charging does not proceed satisfactorily; (3) the system is apt to be influenced by surface properties of a substrate such as a drum.
In order to overcome the above drawbacks, organic photosensitive materials of the positive charge type have been extensively studied. In order to realize the positive charge system, attempts have been heretofore made including
(1) reversed double-layer structures wherein the carrier generation layer and the carrier transport layer are reversed to the case of the negative charge system type; and PA0 (2) single-layer structures wherein various types of carrier generation compounds and carrier transport compounds are both dispersed in a binder resin.
However, the reversed double-layer structure involves the problems such as a lowering of the printing resistance and a poor life characteristic owing to the fact that the carrier generation layer, which has to be substantially thin, is placed on the surface of the photosensitive material. In addition, there arise problems such as of complication in the fabrication process of the double-layer structure and of layer separation. Accordingly, this type of photosensitive material has not been reduced to practice yet.
With the photosensitive materials having the single-layer structure are inferior to the double-layer structure photosensitive materials with respect to the sensitivity, a charging characteristic (i.e. a charging potential gradually drops when the material is repeatedly employed), and a residual potential. Nevertheless, the single-layer structure is advantageous in that if a carrier generation agent and a carrier transport agent are uniformly dispersed throughout the layer, the printing resistance is not lowered immediately after wear of the photosensitive material. This is considered for the reason that the wear of the photosensitive material does not directly influence the photosensitive characteristic. The single-layer structure is easier in fabrication than the double-layer structure. In short, the single-layer structure and the double-layer structure have, respectively, merits and demerits.
We proposed in U.S. patent application Ser. No. 551,538 now U.S. Pat. No. 5,087,540 (corresponding European Patent Application No. 90307677.6) photosensitive materials which comprise an X-type phthalocyanine and/or .tau.-type phthalocyanine dispersed in polymer binders such as polyesters, polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride, polycarbonates, polyvinyl butyral, polyvinyl acetoacetal and the like. The photosensitive material proposed above is convenient for use in the positive charge systems. Further studies revealed that the photosensitive material was not necessarily satisfactory with respect to the life or durability. In fact, when the material is subjected to a repetition cycle test where it is repeatedly used over 10,000 or more printing cycles, the charge voltage, attenuation in the dark and sensitivity are gradually lowered. The reason for this is considered to result from chemical fatigue such as with light and/or ozone generated in the system.